The most widely used forms of photographic elements are those that contain one or more silver halide emulsions. Silver halide emulsions are usually prepared by precipitating silver halide in the form of discrete grains (microcrystals) in an aqueous medium. An organic peptizer is incorporated in the aqueous medium to disperse the grains. Varied forms of hydrophilic colloids are known to be useful as peptizers, but the overwhelming majority of silver halide emulsions employ gelatino-peptizers. A summary of conventional peptizers, including gelatino-peptizers, is provided by Research Disclosure, Vol. 389, September 1996, Item 38957, II. Vehicles, vehicle extenders, vehicle-like addenda and vehicle related addenda, A. Gelatin and hydrophilic colloid peptizers. Research Disclosure is published by Kenneth Mason Publications, Ltd., Dudley House, 12 North St., Emsworth, Hampshire P010 7DQ, England. The term "vehicle" includes both the peptizer used to disperse silver halide grains as they are being formed and the binder used in coating emulsion and processing solution penetrable layers of photographic elements. Gelatin and gelatin derivatives are commonly employed to perform the functions of both peptizer and binder.
The characteristic that is primarily responsible for the dominance of silver halide photography is the image amplification capability of silver halide grains. During imagewise exposure of a silver halide photographic element, incident photons are absorbed by the silver halide grains. When a photon is absorbed, an electron in the silver halide crystal lattice structure of a grain is promoted from a valence band energy level to a higher, conduction band energy level at which it is capable of migrating within the crystal lattice of the grain. When a few conduction band electrons are captured by crystal lattice silver ions in close proximity, a cluster of Ag.sup.o atoms is created, commonly referred to as a latent image site. The latent image site of a grain is capable of catalyzing the overall reduction of silver ions in the grain to Ag.sup.o, a huge amplification of the few original Ag.sup.+ reductions to Ag.sup.o created by imagewise exposure. An imagewise exposed silver halide emulsion is brought into contact with a developer to produce a viewable image. A developer is an aqueous solution containing a developing agent, a reducing agent capable of selectively reducing latent image bearing silver halide grains to Ag.sup.o. Contacting a photographic element with aqueous solutions, including a developer, to produce a viewable image is referred to as photographic processing.
Although many factors come into play in obtaining desirable photographic images, one of the most fundamental is the speed of the photographic element employed. While silver halide photography with its internal amplification mechanism exhibits much higher photographic speeds than other imaging systems, the search for higher photographic speeds in silver halide photography has continued since its inception to the present time, a time period of well over a century. The speed of a photographic element is measured by exposing sample portions of the element at differing levels and then correlating image density following photographic processing. By plotting image density (D) as an ordinate against the log of exposure (E) in lux-seconds, a characteristic curve is generated. The characteristic curve typically contains a portion that exhibits no change in density (minimum density or D.sub.min) as a function of exposure transitioning with increased exposures to a portion in which density increases as a function of increased exposure, often resulting in a linear characteristic curve segment (i.e., .DELTA.D/.DELTA.logE remains constant) transitioning with still higher exposures to a portion in which further exposure does not increase density (maximum density or D.sub.max). Photographic element speeds are usually reported as differences in log E required to produce the same density in compared elements.
Silver halide emulsions possess a native sensitivity to light having wavelengths ranging from the ultraviolet into the blue region of the visible spectrum. Spectral sensitizing dyes are adsorbed to the silver halide grain surfaces to extend sensitivity to longer wavelength portions of the spectrum. A summary of spectral sensitizing dyes is provided by Research Disclosure, Item 38957, cited above, V. Spectral sensitization and desensitization, A. Sensitizing Dyes. The function of a spectral sensitizer is to capture for latent image formation a photon of a wavelength the silver halide grain cannot itself capture.
To increase the speed of silver halide emulsions independent of spectral sensitization, the grain surfaces are treated with chemical sensitizers. A summary of chemical sensitizers is provided by Research Disclosure, Item 38957, cited above, IV. Chemical sensitization.
It has been recently recognized that a further enhancement in photographic speed can be realized by associating with the silver halide grain surfaces a fragmentable electron donating (FED) sensitizer. While no proof of the mechanism of FED sensitization has yet been generated, one plausible explanation is as follows: When, as noted above, photon capture within a grain results in electron promotion from a valence shell to a conduction energy band, a common loss factor is recombination. That is, the promoted electron simply returns to a hole in the valence shell, created by promotion to the conduction band of the same or another electron. When recombination occurs, the energy of the captured photon is dissipated without contributing to latent image formnation. It is believed that the FED sensitizer reduces recombination by donating an electron to fill the hole created by photon capture. Thus, fewer conduction band electrons return to hole sites in valence bands and more electrons are available to participate in latent image formation.
When the FED sensitizer donates an electron to a silver halide grain, it fragments, creating a cation and a free radical. The free radical is a single atom or compound that contains an unpaired valence shell electron and is for that reason highly unstable. If the oxidation potential of the free radical is equal to or more negative than -0.7 volt, the free radical immediately upon formation injects a second electron into the grain to eliminate its unpaired valence shell electron. When the free radical also donates an electron to the grain, it is apparent that absorption of a single photon in the grain has promoted an electron to the conduction band, stimulated the FED sensitizer to donate an electron to file the hole left behind by the promoted electron, thereby reducing hole-electron recombination, and injected a second electron. Thus, the FED sensitizer contributes one or two electrons to the silver grain that contribute directly or indirectly to latent image formation.
FED sensitizers and their utilization for increasing photographic speed are disclosed in Farid et al U.S. Pat. Nos. 5,747,235 and 5,7547,236, and in the following commonly assigned filings: Lenhard et al U.S. Ser. No. 08/739,911, filed Oct. 30, 1996, and Gould et al U.S. Ser. No. 09/118,536, Farid et al U.S. Ser. No. 09/118,552, and Adin et al U.S. Ser. No. 09/118,714, each filed Jun. 25, 1998.
A dramatic increase in photographic speeds in silver halide photography began with the introduction of tabular grain emulsions into silver halide photographic products in 1982. A tabular grain is one which has two parallel major faces that are clearly larger than any other crystal face and which has an aspect ratio of at least 2. The term "aspect ratio" is the ratio of the equivalent circular diameter (ECD) of the grain divided by its thickness (the distance separating the major faces). Tabular grain emulsions are those in which tabular grains account for greater than 50 percent of total grain projected area. Kofron et al U.S. Pat. No. 4,439,520 illustrates the first chemically and spectrally sensitized high aspect ratio (average aspect ratio&gt;8) tabular grain emulsions. In their most commonly used form tabular grain emulsions contain tabular grains that have major faces lying in {111} crystal lattice planes and contain greater than 50 mole percent bromide, based on silver. A summary of tabular grain emulsions is contained in Research Disclosure, Item 38957, cited above, I. Emulsion grains and their preparation, B. Grain morphology, particularly sub-paragraphs (1) and (3).
The use of cationic starch as a peptizer for the precipitation of high bromide {111} tabular grain emulsions is taught by Maskasky U.S. Pat. Nos. 5,604,085, 5,620,840, 5,667,955, 5,691,131, and 5,733,718. Oxidized cationic starches are advantageous in exhibiting lower levels of viscosity than gelatino-peptizers. This facilitates mixing. Under comparable levels of chemical sensitization higher photographic speeds can be realized using cationic starch peptizers. Alternatively, speeds equal to those obtained using gelatino-peptizers can be achieved at lower precipitation and/or sensitization temperatures, thereby avoiding unwanted grain ripening.
When silver halide grains are developed, the light exposed (as opposed to the non-exposed) silver halide grains are selectively reduced with a developing agent. During this reaction silver halide is reduced to silver, and the developing agent is oxidized. When it is desired to form a dye image, the developing agent is usually chosen to be a color developing agent, which is a developing agent that, following oxidization, reacts to complete an image dye chromophore. The most common route to image dye formation is the reaction of an image dye-forming coupler with a para-phenylenediaamine color developing agent, which is a para-phenylenediamine in which at least one of the amine groups is unsubstituted. Dye chromophore formation occurs when one or two quinonediimine molecules (each of which requires two molecules of oxidized para-phenylenediamine color developing agent to produce) reacts with the image dye-forming coupler. When an image dye-forming coupler requires two quinonediimine molecules to form an image dye molecule, the image dye-forming coupler is said to be a four equivalent coupler, since four molecules of color developing agent must be oxidized to result in each molecule of image dye. Two equivalent coupler image dye-forming couplers are those that spontaneously split off an anionic (e.g., halogen) or low pKa leaving group (e.g., phenol or heterocycle) under the conditions of development and therefore react with a single quinonediimine molecule to form an image dye molecule. These mechanisms of image dye formation are textbook knowledge, as illustrated by the Color Photography topic in The Kirk-Othmer Encyclopedia of Chemical Technology, John Wiley and Sons, New York, 1993; Vol. 6.
Since the molar ratio of image dye produced to developed silver is lower when a four equivalent image dye-forming coupler is employed than when a two equivalent image dye-forming coupler is employed and since the photographic speeds of color photographic elements are compared by measuring the exposure difference required to reach a reference image dye density, it is apparent that otherwise comparable color photographic elements containing two equivalent image dye-forming couplers exhibit higher imaging speeds than those that contain four equivalent image dye-forming couplers. This recognition led to investigation of one equivalent image dye-forming couplers. One equivalent image dye-forming couplers are similar to two equivalent image dye-forming couplers in that only one quinonediimine molecule is required to form an image dye molecule. One equivalent couplers differ from two equivalent couplers in that the leaving group that is split off prior to coupling itself supplies a molecule of image dye which is in addition to the molecule of image dye produced by coupling. Hence, reduction of two molecules of silver halide to silver produces two molecules of oxidized para-phenylenediamine color developing, which produce one molecule of quinonediimine that reacts with a one equivalent coupler to produce two image dye molecules. Hence, in theory (ignoring second order reaction inefficiencies) there is a one to one molar ratio of developed silver to image dye. The unique requirements imposed by dye chromophore containing leaving groups in one equivalent image dye-forming couplers have limited their application, with two and four equivalent structures forming the overwhelming majority of image dye-forming couplers. One equivalent image dye-forming couplers are described in Mooberry et al U.S. Pat. Nos. 4,840,884, 5,447,819 and 5,457,004.
In the last two decades enhancements in dye images attributable to the incorporation of dye image modifying couplers have become common. These couplers, which often do not form an image dye on coupling, can be relied upon for immediate or timed release of photographically useful fragments, such as development accelerators, development inhibitors, bleach accelerators, bleach inhibitors, developing agents (e.g., competing or auxiliary developing agents), silver complexing agents, fixing agents, toners, hardeners, tanning agents, antistain agents, stabilizers, antifoggants, competing couplers, and chemical or spectral sensitizers or desensitizers.
A summary of couplers is provided by Research Disclosure, Item 38957, cited above, X. Dye image formers and modifiers, particularly B. Image-dye-forming couplers and C. Image dye modifiers.